The use of radiation-sensitive silver halide emulsions for medical diagnostic imaging can be traced to Roentgen's discovery of X-radiation by the inadvertent exposure of a silver halide film. Eastman Kodak Company then introduced its first product specifically that was intended to be exposed by X-radiation in 1913.
In conventional medical diagnostic imaging the object is to obtain an image of a patient's internal anatomy with as little X-radiation exposure as possible. The fastest imaging speeds are realized by mounting a dual-coated radiographic element between a pair of fluorescent intensifying screens for imagewise exposure. About 5% or less of the exposing X-radiation passing through the patient is adsorbed directly by the latent image forming silver halide emulsion layers within the dual-coated radiographic element. Most of the X-radiation that participates in image formation is absorbed by phosphor particles within the fluorescent screens. This stimulates light emission that is more readily absorbed by the silver halide emulsion layers of the radiographic element.
Examples of radiographic element constructions for medical diagnostic purposes are provided by U.S. Pat. No. 4,425,425 (Abbott et al.) and U.S. Pat. No. 4,425,426 (Abbott et al.), U.S. Pat. No. 4,414,310 (Dickerson), U.S. Pat. No. 4,803,150 (Kelly et al.), U.S. Pat. No. 4,900,652 (Kelly et al.), U.S. Pat. No. 5,252,442 (Tsaur et al.), and Research Disclosure, Vol. 184, August 1979, Item 18431.
While the necessity of limiting patient exposure to high levels of X-radiation was quickly appreciated, the question of patient exposure to even low levels of X-radiation emerged gradually. The separate development of soft tissue radiography, which requires much lower levels of X-radiation, can be illustrated by mammography. The first intensifying screen-film combination (imaging assembly) for mammography was introduced to the public in the early 1970's. Mammography film generally contains a single silver halide emulsion layer and is exposed by a single intensifying screen, usually interposed between the film and the source of X-radiation. Mammography utilizes low energy X-radiation, that is radiation that is predominantly of an energy level less than 40 keV.
U.S. Pat. No. 6,033,840 (Dickerson) and U.S. Pat. No. 6,037,112 (Dickerson) describe asymmetric imaging elements and processing methods for imaging soft tissue.
Problem to be Solved
In mammography, as in many forms of soft tissue radiography, pathological features that are to be identified are often quite small and not much different in density than surrounding healthy tissue. Thus, the use of films with relatively high average contrast (in the range of from 2.5 to 3.5) over a density range of from 0.25 to 2.0 is typical. Limiting the amount of X-radiation requires higher absorption of the X-radiation by the intensifying screen and lower X-radiation exposure of the film. This can contribute to loss of image sharpness and contrast. Thus mammography is a very difficult task in medical radiography.
Radiographic imaging of soft tissue as in mammography is usually carried out using low peak voltage (kVp), for example, 28 kVp, from the imaging equipment to maximize image sharpness. However, the consequence of low peak voltage is higher patient dose.
Moreover, radiographic imaging of soft tissue is usually carried out using X-ray equipment that includes an X-ray tube with a rotating anode. The anode is the “source” of the X-radiation that is created when electrons interact with the electrons or nuclei of the metallic atoms in the anode. To maximize image quality, molybdenum anodes are generally used in such equipment. Rhodium anodes are also known in the art particularly for lowering patient exposure to radiation, but in the case of mammography, poorer image quality is usually results when they are used
There remains a need in mammography for a way to minimize patient exposure to radiation while providing optimal radiographic image quality such as image contrast.